New Perspectives on the Critical Velocity for Smoke Control

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Fourth International Symposium on Tunnel Safety and Security, Frankfurt am Main, Germany, March 17-19, 2010 Fire Size (MW) Distance Upstream of Fire (m) Distance Downstream of Fire (m) 5 - - 20 10 40 50 20 80 100 30 120 Table 1: Distances over which Jet Fans may be considered as destroyed during Fire [12] It is certainly possible that a tunnel ventilation system may ‘blow too hard’ and actually increase the fire heat release rate from a burning vehicle. Fig. 2 depicts the possible enhancement factors ‘k’ of peak fire heat release rates from burning heavy goods vehicles (HGVs), as a function of tunnel ventilation velocity. Values of ‘k’ above unity indicate an enhancement in the fire heat release rate. The two curves shown are for single-lane and dual-lane tunnels The exact physical processes involved in the fire heat release rate enhancement are not well understood, but could involve providing additional oxygen to partially shielded fires within a burning HGV. Apart from enhancing the peak fire heat release rate, there is also the possibility of a higher fire growth rate within the burning vehicle, and of fire spread to other vehicles within the tunnel. Fig. 2: Fire Heat Release Rate Enhancement Factors (after Carvel et al [11]) In order to avoid the drawbacks of excessive air velocities and possible fire enhancement, a control system may be required to monitor the tunnel air velocities and feed these air velocities back to an automatic control system. Such a ventilation control system is considered standard in a number of European countries including Switzerland and Austria, but not in the United Kingdom, for example. The Austrian design standard RVS 09.02.31 [13] specifies that the longitudinal ventilation system should reduce the air velocity in unidirectional traffic to between 1.5 m/s to 2 m/s, and for bidirectional traffic to a value between 1 m/s and 1.5 m/s. The Austrian design standard does not however mention any requirement to meet the critical air velocity for smoke control. For unidirectional traffic without congestion, the German RABT guidelines [14] require a minimum velocity of the air flow exceeding the critical velocity for smoke control. The World Road Association’s report on “Operational Strategies for Emergency Ventilation” [15] provides three different cases for operational control of the tunnel air velocity in case of an incident, as summarised in Table 2. Although the text of the report does refer to the critical air velocity for smoke control, it does counsel that while high flowrates may have the advantage of reducing temperature and decreasing toxicity, they may lead to higher fire heat release rates and will completely destroy any smoke stratification. 424
Fourth International Symposium on Tunnel Safety and Security, Frankfurt am Main, Germany, March 17-19, 2010 CASE A B C TRAFFIC PRIOR TO INCIDENT Unidirectional traffic without traffic congestion Unidirectional traffic with traffic congestion Bidirectional traffic PRINCIPLE FOR LONGITUDINAL VENTILATION Flow velocities in the direction of traffic to prevent or at least minimize backlayering of smoke Relatively low flow velocities (e.g. 1.2 ± 0.2 m/s) in the direction of traffic in order to minimize flow spread upstream, allow smoke stratification, support dilution of toxic gases and enable people to escape. Relatively low flow velocities should be maintained, to avoid flow reversal unless circumstances dictate otherwise (for example fires near portals), allow smoke stratification and enable people to escape in both directions. Note: reference should also be made to National Guidelines, the EU Directive or similar for further advice on the design aspects relating to tunnel length, ventilation objectives and design etc. Table 2: Strategies for Smoke Control with Longitudinal Ventilation Systems [15] Any tunnel ventilation control system is critically dependent on the quality of the real-time air velocity measurements provided during an incident. Key issues to consider in any ventilation control system design include: the redundancy of air velocity sensors and their locations; the effect of hot smoke movement through the air velocity sensors, and the control algorithm to interpret significant differences in the measurements provided by the sensors, including elimination of false readings. None of these issues is insurmountable, however – perfectly feasible control system designs are available that address these challenges. As our understanding of the risks of fire enhancement and smoke destratification due to excessive ventilation increases, it is likely that feedback ventilation control systems will become specified in tunnels as a matter of course. This is particularly true in the case of road tunnels, where the number of independent parameters that may affect the air velocity are generally greater than in rail or metro tunnels. CONCLUSIONS The concept of a critical velocity to control the upstream movement of smoke in tunnels has been available for many decades. This paper represents an effort towards quantifying the critical velocity, by providing analytical solutions for its value in tunnels and cross-passages. However, the paper also proposes caution in how the concept of a critical velocity is interpreted and applied in tunnel ventilation designs, since there are a number of drawbacks in the generation of high air velocities in tunnels during a fire emergency. These drawbacks include a possible enhancement of vehicle fires, and loss of smoke stratification. Careful engineering design, backed up by references to national and international guidelines, is therefore called for in the interpretation and application of the critical air velocity concept. REFERENCES 1. Thomas, P. H., “Movement of smoke in horizontal corridors against an air flow”, The Institution of Fire Engineers Quarterly, 30(77), pp.45-53, 1970. 2. Lee, C.K., Hwang, C.C., Singer, J.M. und Chauken, R.F.,“Influence of Passageway Fires on Ventilation Flows", Second International Mine Ventilation Congress, Reno, Nevada, 4-8 November 1979. 3. Kennedy, W.D. (1996) "Critical Velocity : Past, Present and Future", One Day Seminar on Smoke and Critical Velocity in Tunnels, ITC, 2 April 1996. 4. Subway Environmental Design Handbook, Volume II, SES Version 4.1, Part I User’s Manual, February 2002. 425
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